Transponders are electronic devices incorporated into secure documents such as “smart cards” and “electronic passports” using RFID (radio frequency identification) technology.
The transponder (or inlay, or chip card) itself generally comprises (includes):                a substrate (or inlay substrate) which may comprise a sheet of a synthetic material;        a chip (or chip module, or chip unit) installed on the substrate (or in a recess in the surface of the substrate) and having terminals (or contact surfaces, or pads); and        an antenna wire (or conductor) mounted on the substrate, formed with “turns” as a flat coil and connected (bonded) by its ends or end portions to the terminals of the chip module. (In some of the drawings presented herein, only one end or end portion of the antenna wire may be shown, for illustrative clarity, particularly in the cross-sectional views.)        
The inlay substrate may comprise one or more layers of PVC, Polycarbonate (PC), polyethylene (PE), PET (doped PE), PETE (derivative of PE), TYVEK, Teslin™, Paper or Cotton/Noil, and the like. For example, a single layer of uncoated Teslin™, with a thickness of 356 microns. In the main hereinafter, inlay substrates comprising Teslin™ or polycarbonate (PC) will be described.
Teslin™ is a synthetic printing media, manufactured by PPG Industries. Teslin™ is a waterproof synthetic material that works well with an Inkjet printer, Laser printer, or Thermal printer. Teslin™ is also single-layer, uncoated film, and extremely strong. In fact, the strength of the lamination peel of a Teslin™ sheet is 2-4 times stronger than other coated synthetic and coated papers. Teslin™ comes in the sizes of 7 mil to 18 mil, though only sizes 10 mil and 14 mil are sized at 8.5″ by 11″, for printing with most consumer printers. Also available are perforated versions of Teslin, specifically, 2up, 6up, and 8up. Teslin™ is a microporous polymer. Polycarbonate (PC) is typically used for national ID cards, and also as the material in certain passports (such as for the Datapage, in contrast to the e-Cover).
The inlay substrate may have an area designated as a “transponder site” whereat the chip module will be installed. (A recess in the inlay substrate may constitute the transponder site.) The transponder site may itself have two areas designated as “terminals areas” corresponding in position to the two terminals of the chip module which will be installed at the transponder site. (The transponder site and terminal areas are generally geometric abstractions, the chip module and terminals are physical elements.) Hence, it should be understood that, where applicable, the terms (and reference numerals for) “transponder site” and “chip module” may be used interchangeably, and that the terms “terminal areas” and “terminals” may similarly be used interchangeably.
In the main hereinafter, RFID chips incorporated into chip modules will be described. The chip module may be a leadframe-type chip module comprising an RFID chip encapsulated by a mold mass and supported by and connected to a leadframe having two terminal areas.                the mold mass may be approximately 240 μm thick and 5 mm wide        the leadframe may be approximately 80 μm thick and 8 mm wide.        
The chip module may be disposed in a recess extending into the surface of the substrate measuring for example 5.5 mm wide×8.5 mm high (generally the recess is only slightly larger than the chip module to allow some clearance during installation, while maintaining good registration).
The recess for receiving the chip module extends into the inlay substrate from a “top” surface thereof, and may be a “window” type recess extending completely through the inlay substrate to a “bottom” surface thereof, or the recess may be a “pocket” type recess extending only partially through the inlay substrate towards the bottom surface thereof.
The recess may have a “straight” profile—in other words, substantially constant cross-dimension through (or into) the inlay substrate. Or, the recess may have a “stepped” profile, including a larger cross-dimension at the top surface of the substrate than at (or towards) the bottom surface of the inlay substrate. The recess is generally sized and shaped to accommodate the size and shape of the chip module being disposed therein. The term “cavity” may be used interchangeably with “recess”. A stepped recess profile is commonly used to accommodate a leadframe module, since the leadframe is typically wider (8-10 mm) than the mold mass (4-6 mm) of the chip module.
The antenna wire can be self-bonding copper wire or partially coated self-bonding copper wire, enamel copper wire or partially coated enamel wire, silver coated copper wire, un-insulated wire, aluminum wire, doped copper wire or litz wire.
The conventional method of mounting the wire is using a sonotrode tool which vibrates, feeds the wire out of a capillary, and embeds it into the surface of the substrate. Examples of embedding a wire in a substrate, in the form of a flat coil, and an ultrasonic tool for performing the embedding (and a discussion of bonding), may be found in U.S. Pat. No. 6,698,089 (refer, for example, to FIGS. 1, 2, 4, 5, 12 and 13 of the patent). See also FIGS. 1 and 2 of U.S. Pat. No. 6,233,818. Both of these patents are incorporated by reference herein. It is also known that a coated, self-bonding wire will stick to a synthetic (e.g., plastic) substrate because when vibrated sufficiently to soften (make sticky) the coating and the substrate.
The conventional method for connecting the ends or end portions of the antenna wire to the terminals (or “terminal areas”) of the chip module is by means of thermo compression (TC) bonding. This method makes use of heat by passing pulses of electric current through a thermode and simultaneously applying pressure to cause a diffusion process between the wire and the leadframe of the chip module.
FIGS. 1A and 1B illustrate an example of a prior art technique, such as is disclosed in U.S. Pat. No. 6,233,818 for mounting an antenna wire to an inlay substrate and connecting the antenna wire to a chip module installed in a recess in the inlay substrate.
An inlay sheet 100 is a large inlay substrate which may have a plurality of transponder areas (or sites) 102, a one of which is shown in some detail. Typically, several transponders (or transponder sites) are fabricated on a single inlay sheet.
A recess 106 is formed in the inlay substrate 102 for receiving a leadframe type chip module 108, positioned as shown, with the mold mass 112 situated below the leadframe 114. The leadframe 114 of the chip module 108 has two terminal areas 108a and 108b. An antenna wire 110 having two ends (or end portions) 110a and 110b is mounted on the substrate and connected to the terminal areas 108a and 108b of the chip module 108.
The wire may is mounted to the substrate using an ultrasonic embedding tool such as a sonotrode having a capillary 116. Mounting the antenna wire may proceed as follows:                using the sonotrode, embed the wire a short distance, between the points “a” and “b” near a first terminal of the chip module. (embedding is indicated by the symbols “x”)        stop embedding (raise the sonotrode), and pass over the first terminal of the chip module, between the points “b” and “c”.        lower the sonotrode and resume embedding at the point “c”, and form the turns of the antenna between the points “c” and “d” (embedding is indicated by the symbols “x”)                    there may be for example 4 or 5 turns, and the overall length of the antenna wire may be 104 cm            notice that in forming the turns of the antenna, the wire may need to cross over itself, thus requiring an insulated wire. However, in some cases, the antenna wire does not need to cross over itself. See, for example, FIG. 4 of U.S. Pat. No. 6,698,089.                        after approaching near the second terminal of the chip module, stop embedding and pass over the second terminal of the chip module, between the points “d” and “e”.        resume embedding a short distance on the opposite side of the chip module, between the points “e” and “f”.        
The embedding process may be discontinuous, at several points, rather than continuous.
In a next stage of the process, the “connection” portions of the wire passing over the terminal areas are interconnected to the terminal areas of the RFID chip, typically by means of thermo compression bonding. A thermode 118 for performing bonding is illustrated. It is known to remove insulation from the connection portions of the antenna wire to improve bonding.
In the case of Teslin (synthetic paper), a normal insulated wire would not properly embed into the material, it would detach. Therefore, it is known to use self-bonding wire which attaches to the material with a slight penetration of the wire in the material.
A self-bonding (or self-adhering) wire may comprise                a metallic core (typically, but not necessarily round in cross-section) comprising copper, aluminum, doped copper, gold, or Litz wire, and may have a diameter of 0.010-0.50 mm        a first coating or “base coat” comprising modified polyurethane, and having a thickness of only a few microns        a second coating comprising polyvinylbutyral or polyamide, and having a thickness of only a few microns.        
The transponder thus formed on the inlay substrate may be incorporated, for example, in an electronic passport cover. The material for the cover layer of the passport may be a cloth product, with chemistry in the coatings and a leather-like appearance to the cloth, such as by Holliston Inc. (905 Holliston Mills Road, Church Hill, Tenn. 37642; www.holliston.com)
FIG. 1C shows a passport cover having a cover layer and an inlay substrate. The cover layer is laminated (joined) to the inlay substrate using a polyurethane hot melt adhesive, such as approximately 50-80 μm thick. Prior to the adhesive process, the inlay substrate may be pre-pressed to ensure that the antenna wire does not protrude over (extend above) the surface of the Teslin™ substrate, in other words, to ensure that the antenna wire is fully embedded in the inlay substrate.